Apparatus and method for concealing erased periodic signal data

ABSTRACT

Circuitry and a method compensate the erasure of speech signal data or similar periodic signal data, by substitution using past periodic signal data input. After a predetermined number of latest periodic signal data have been saved, whether or not an erasure occurs is determined with every periodic signal data sequence, which is a unit of processing. When an erasure occurs, one of periodic signal data sequences saved, which lies in a determined segment to be used, is used to generate synthetic data for substitution. The position of the segment to be used is determined such that when the erasure continues over units of processing, the position sequentially varies gradually for each processing units.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to compensating circuitry for compensatingerased periodic signal data and a compensating method therefor, and isapplicable to, e.g. the compensation of the erasure of a speech signal.

2. Description of the Background Art

While speech communication over Internet or similar communicationnetwork is extensively used today, it is likely that speech sent over anetwork is partly erased or lost, resulting in the degradation speechquality. To improve degraded speech quality, a method taught in ITU-T(International Telecommunication Union-Telecommunication StandardizationSector) Recommendation G.711 Appendix I is available.

In accordance with the method taught in the above document, a codedspeech signal arrived over a network is decoded by a speech decoder andthen input to a compensating circuitry. The compensating circuitrymonitors the input decoded speech signal on a speech frame basis, whichis the unit of speech signal decoding, and executes compensation everytime the erasure of speech occurs. More specifically, when any speech ismissing, the compensating circuitry determines a period or waveformfrequency around the time when an erasure has occurred on the basis ofspeech data stored in, e.g. a memory included in the circuitry, andreceived just before the above time. Subsequently, the compensatingcircuitry reads out the speech data stored in the memory, andsubstitutes the data for a frame which the erasure is associated withand requires speech signal substitution, such that the start phase ofthe frame coincides with the end phase of the immediately precedingframe to thereby maintain continuity in waveform period.

The memory of the compensating circuitry has a storage capacity largeenough to store speech data over, e.g. up to three consecutive waveformperiods, so that an undesirable tone ascribable to a single continuouswaveform can be obviated by use of the three waveform periods of speechdata. Should only one waveform period of speech data be saved, it wouldcause unnecessary tones to generate when repeatedly used forsubstitution.

However, saving up to three waveform periods of speech data for thecompensation of an erasure is not practicable without scaling up thememory and access configuration thereof and therefore the entirecompensating circuitry. In addition, when erasure frames occursuccessively, the section for use in forming substitution data underspeech was expanded by a multiple of the waveform period. Therefore,when erasure frames come successively, data under speech available forforming substitution data would resultantly be obtained from the longersection. Accordingly, the naturality in tonal fluctuation of asubstituted speech may be spoiled.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide compensatingcircuitry free from the drawback stated above and capable of concealingthe partial erasure of a periodic signal and a compensating methodtherefor.

In accordance with the present invention, compensating circuitry forsubstituting past periodic signal data input for erased periodic signaldata includes a past data saving circuit for saving a predeterminednumber of latest periodic signal data input. A decision circuitdetermines whether or not an erasure occurs with every periodic signaldata sequence, which is a unit of processing. When an erasure occurs, asubstituting circuit uses, among periodic signal data sequences saved inthe past data saving circuit, a periodic signal data sequence lying in apredetermined segment to be used to generate synthetic data forsubstitution or interpolation. When the erasure continues over aplurality of units of processing, a position controller determines aposition of the segment to be used such that the position varies foreach of the units of processing.

Also, in accordance with the present invention, a compensating method ofsubstituting past periodic signal data input for erased periodic signaldata begins with a past data saving step of saving a predeterminednumber of latest periodic signal data input. Whether or not an erasureoccurs is determined with every periodic signal data sequence, which isa unit of processing. When an erasure occurs, a periodic signal datasequence lying in a predetermined segment to be used is used amongperiodic signal data sequences saved in the past data saving step togenerate synthetic data for substitution or interpolation. Further, whenthe erasure continues over a plurality of units of processing, aposition of the segment to be used is determined such that the positionvaries for each of the units of processing.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the present invention will become moreapparent from consideration of the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 is a schematic block diagram showing erasure compensatingcircuitry embodying the present invention;

FIG. 2 is a graph plotting a specific result of processing executed byan autocorrelation calculating circuit included in the illustrativeembodiment;

FIG. 3 demonstrates a procedure to be executed by the illustrativeembodiment for generating synthetic speech data for substitution;

FIG. 4 shows a procedure to be also executed by the illustrativeembodiment for determining a active segment, which delimits the range ofpast speech data to be used for substitution;

FIG. 5 shows an active segment determining procedure executed with analternative embodiment of the present invention;

FIG. 6 shows an active segment determining procedure executed withanother alternative embodiment of the present invention;

FIG. 7 shows an active segment determining procedure executed with afurther alternative embodiment of the present invention; and

FIG. 8 shows a conventional speech erasure compensating method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawings, speech erasure compensatingcircuitry embodying the present invention is applied to a speech signalby way of example. It is to be noted that the circuitry shown in FIG. 1may be implemented entirely by hardware or partly by software so long asit can achieve functions to be described hereinafter.

As shown in FIG. 1, the speech erasure compensating circuitry, generally10, includes a speech substituting circuit 12, two data memories (A) 14and (B) 16, an erasure decision circuit 18, an autocorrelationcalculating circuit 20 for detecting a period of speech data, and asubstitution controller 22 interconnected as illustrated. The circuitry10 also includes a speech decoder 26, which is adapted to decode speechdata received over a network on its input port 30 and has its outputport 24 connected to the input of the speech substituting circuit 12.

Receiving decoded speech data from the speech decoder 26 via the input24, the speech substituting circuit 12 simply passes the speech datatherethrough if the speech data are not erased. If the speech data areerased, the speech substituting circuit 12 performs substitution orinterpolation by using speech data stored in the data memory 16 underthe control of the substitution controller 22.

Non-erased speech data, sometimes referred to as complete speech data inthe context, output from the speech decoder 26 are input to the datamemory 14 via the speech substituting circuit 12 and used for thecompensation of an erasure. In the illustrative embodiment, the durationof speech data to be saved in the data memory 14 is shorter than withthe conventional circuitry. For example, the data memory 14 has itsstorage capacity just large enough to store a few waveform periods ofspeech data at most. The waveform period of speech data lies in therange of 5 to 15 milliseconds although it can, of course, be suitablyselected by a designer. The data memory 14 has its output 32 connectedto the other data memory 16.

When the substitution of speech data should be executed, the speech datastored in the data memory 14 are copied into the data memory 16. Thisallows, even when speech data stored in the data memory 14 are updated,speech data having appeared just before substitution to be preserved inthe data memory 16.

The erasure decision circuit 18 determines whether or not speech dataare erased. For example, if the frame number representative of the orderof speech frames having arrived is not obtained, if the frame numberobtained is the same as the past frame number, or if the frame number isobtained but speech data associated therewith cannot be decoded due to,e.g. an error detected, then the erasure decision circuit 18 determinesthat the speech data of the frame designated by the frame number inquestion are missing. The function of the erasure decision circuit 18may be assigned to the speech decoder 26, if desired. In any case, theerasure decision circuit 18 forms part of the speech erasurecompensating circuitry 10. The result of decision output from theerasure decision circuit 18 is delivered to the substitution controller22 and autocorrelation calculating circuit 20.

When speech data are missing, the autocorrelation calculating circuit 20calculates, under the control of the substitution controller 22, theautocorrelation value of a speech data sequence saved in the data memory14 and then produces a waveform period 34 and a shift period 36 from theautocorrelation value, thereby detecting synchronization. The waveformand shift periods 34 and 36 thus produced are fed to the substitutioncontroller 22.

FIG. 2 is a graph plotting a specific result of calculation output fromthe autocorrelation calculating circuit 20; the abscissa indicates theamount of shift while the ordinate indicates an autocorrelationcorresponding to the amount of shift. A waveform period refers toconventional basic information on a period particular to a speech datasequence. In the illustrative embodiment, the waveform period of speechdata, generally ranging from 5 to 15 milliseconds, refers to the amountof shift having the maximum autocorrelation within the above range ofcourse, the range of waveform period search may be broader or narrowerthan the above range, if desired.

On the other hand, a shift period is detected as information defining aspeech data segment in the data memory 16 and is used to interpolate,when speech data are missing over two or more consecutive frames, speechdata in frames that follow the second frame. A shift period isimplemented by the amount of shift at the maximum peak autocorrelationvalue lying in a shift amount narrower than the waveform period. A shiftperiod may be defined from another point of view. For example, anadditional condition that the amount of shift corresponds to a peakautocorrelation value lying in the range of one-fourth to three-fourthsof the waveform period may be used for decision.

Generally, a speech signal consists of a plurality of frequencycomponents overlapping each other, so that a plurality of peakautocorrelation values appear even outside the waveform period. One ofsuch a plurality of peak autocorrelation values that satisfies thepreselected condition is used as a shift period.

The waveform and shift periods may be determined by any suitable methodother than the method using an autocorrelation stated above, e.g. amethod using frequency analysis.

Referring again to FIG. 1, the substitution controller 22 controls theentire compensating circuitry 10 to substitute speech data for an erasedframe. The autocorrelation calculating circuit 20 uses the pastpredetermined number of speech data and the latest complete speech dataas a reference to produce an autocorrelation. This means that thecompensating circuitry 10 knows the last phase of a speech data sequencehaving appeared just before a frame in which speech data are missing.

The operation of the compensating circuit 10 with the aboveconfiguration will be described with reference made to FIGS. 3A through3D and 4 as well. In the following description, the storage areas of thedata memories 14 and 16 will be referred to as buffers A and B,respectively. Overlap Add processing described in ITU-T G.711 may beexecuted although not shown or described specifically.

While speech data input to the compensating circuitry 10 are writteninto the buffer A, as shown in FIG. 3, part [A], the content of thebuffer A is updated every frame. The capacity of the buffer A may be,but not limited to, a few times as large as the maximum waveform periodlength.

When a frame whose speech data are erased occurs, the waveform and shiftperiods stated earlier are calculated from a speech data sequence savedin the buffer A and then memorized until the erasure of speech dataends. Further, the speech data sequence stored in the buffer A arecopied into the buffer B in order to produce synthetic speech data forsubstitution and are held in the buffer B until the erasure ends. Atthis instant, one frame of synthetic speech data are produced from onewaveform period of speech data, so that reconstructed waveform data orspeech data are output.

First, a procedure for producing synthetic speech data for substitutionwill be described on the assumption that speech data are missing in onlyone frame. In this case, speech data to be used for substitution extendfrom a point just before an erasure occurs to a point one waveformperiod before the above point. This segment will sometimes be referredto as an active segment. As shown in FIG. 3, part [B], speech datahaving appeared one waveform period before the beginning of an erasureare used as the start point (311) of speech data for substitution. Toproduce speech data for substitution, speech data are used, extendingfrom the start point (311) to the right end (313) of one waveform periodIf the speech data for substitution, labeled 301, are short of one frameeven at the right end (313) of one waveform period, then the procedurereturns to the left end (314).

When the procedure returns from the right end (313) to the left end(314) for producing speech data for substitution, it causes an segmentat the left side of the right end (313) and an segment at the left sideof the left end 314, corresponding to one-fourth of a period each, tooverlap each other, thereby effecting continuous transition from theright end (313) to the left end (314). This overlap scheme is defined as“overlap add” in ITU-T Recommendation G.711. Likewise, an segment justbefore the erasure of speech and an segment at the left side of thefirst frame, corresponding to one-fourth of a period each, are caused tooverlap each other, so that continuous transition occurs from the speechdata just before the erasure to the synthetic speech data. The overlapscheme based on ITU-T Recommendation G.711 is only illustrative and maybe replaced with any other scheme capable of continuously connectingspeech waveforms.

How synthetic speech data for substitution are produced when speech iserased over two consecutive frames will be described hereinafter. Forthe first frame where speech data are missing, synthetic speech data areproduced in the same fashion as when speech is missing in only oneframe. Synthetic speech data for the second frame where speech data arealso missing are generated by the following procedure.

First, as shown in FIG. 3, part [C], the active segment is shifted fromthe position used for the substitution of the first frame to the left byone shift period (320). Speech data (302) for substitution are producedfrom the resulting new active segment (326). The active segment (326)has a start point (321) determined in accordance with the following way.

The end point of the active segment used for the first frame is assumedto be a temporary start point (325), which is coincident with the endpoint (312) shown in FIG. 3, [B]. If the temporary start point (325)lies in the current active segment (326) between the left end (324) andthe right end (323), the temporary start point (325) is used as anactual start point. If the temporary start point (325) does not lie inthe current active segment (326), a point in an segment (326) shiftedfrom the temporary start point (325) to the left by one waveform periodis determined to be an actual start point (321). The generation ofspeech data for the second erased frame begins with speech datapositioned at such an actual start point.

Again, an segment at the right side of the end point (312) of the firstframe and an segment at the right side of the start point (321) of thesecond frame, corresponding to one-fourth of a period each, are causedto overlap each other so as to insure continuous transition from thespeech data of the first frame to that of the second frame. The overlapscheme based on ITU-T Recommendation G.711 may be replaced with anyother scheme capable of continuously connecting speech waveforms, asstated earlier.

When speech data are missing over three or more consecutive frames,synthetic speech data to be substituted in the third frame are producedin the same fashion as the synthetic speech data substituted in thesecond frame, i.e. by determining an active segment based on the shiftperiod, determining a start point within the active segment, and thenproducing speech data for substitution, see FIG. 3, [D].

It should be noted that synthetic speech data to be substituted in thesecond and successive erased frames each are continuously attenuatedbefore they are output. When the attenuation ratio exceeds 100%, ZEROsare output as speech data.

As for the third and successive frames, too, the active segment issequentially shifted to the left frame by frame by one shift period at atime, as stated above. It is therefore likely that the active segmentshifted to the left by one shift period exceeds the range of the bufferB. In such a case, synthetic speech data for substitution are producedby a procedure to be described with reference to FIG. 4 hereinafter.

FIG. 4 demonstrates the variation of the active segment in the buffer B.As shown, for the second and successive frames, an active segment (B1)assigned to the first frame on the basis of the waveform period issequentially shifted to active segments (B2) and (B3) frame by frame byone shift period at a time. As a result, it may occur that an activesegment (341), following the active segment (B3), includes the left sideof the left end (351) of the buffer B, as represented by an activesegment (B4). In this case, the active segment (341) is shifted to theright by one waveform period, and the resulting segment is used as anactive segment (342) for the generation of synthetic speech data.

More specifically, the active segment (342) has a start point (344)determined with the following manner. If a temporary start point (343),coincident with the end point (330) of the previous frame, lies in ansegment (342), it is determined to be the start point. If the temporarystart point 343 does not lie in the segment (342), the active segment(342) is sequentially shifted to the right by one waveform period at atime until the end point (330) of the previous frame enters the segment342. When speech is missing even in the other frames to follow, activesegments (B5) and (B6) each are shifted to the left by one shift periodand then shifted, if exceeding the range of the buffer B, to the rightby one waveform period.

When a complete speech data sequence again appears after the erasure,overlap processing based on the ITU-T G.711 standard should preferablybe executed for insuring continuous transition from synthetic,substituted speech data to real speech data. At this instant, theoverlap processing uses the right side of the end point of the lastsynthetic speech data and the start point of the real speech data. Theabove overlap processing may, of course, be replaced with any otherprocessing capable of implementing a continuous transition.

As stated above, the illustrative embodiment produces synthetic speechdata for substitution by calculating two different periods, i.e. awaveform period and a shift period and shifts frame by frame an activesegment over which the past speech data are used on the basis of thecalculated shift period. The active segment therefore sequentially moveswhile overlapping the previous active segment. This allows a memory witha small capacity to suffice for saving the past speech data andtherefore reduces the scale of the entire compensating circuitry.

Of course, the illustrative embodiment is similarly practicable with theconventional memory having a large capacity, in which case a number ofwaveform data or active segments can be used. This allows syntheticspeech data to include many kinds of variations and therefore soundnatural. Stated in another way, with circuitry capable of using a largermemory capacity, it is possible to generate speech data that includemore variations and therefore sound more natural.

Further, the illustrative embodiment shifts the active segment graduallyand can therefore obviate the continuous generation of a single waveformundesirable as reconstructed speech. It follows that natural speech datacan be substituted that obviate an unnatural feeling as to the auditorysense. Moreover, the illustrative embodiment determines the shift widthof the active segment by use of the shift period derived from thewaveform period, thereby insuring continuity of speech data.

An alternative embodiment of the speech erasure compensating circuitryin accordance with the present invention will be described withreference to FIG. 5. Because the illustrative embodiment is essentiallysimilar to the previous embodiment, let the following descriptionconcentrate on a procedure unique to the illustrative embodiment.Briefly, the illustrative embodiment differs from the previousembodiment as to the method of determining an active segment when theactive segment shifted to the left by the shift period exceeds the rangeof the buffer B.

FIG. 5 shows the buffer B and how the active segment varies in theillustrative embodiment. Active segments (B1) through (B3) shown in FIG.5 are identical with the active segments (B1) through (B3) shown in FIG.4. As shown in FIG. 5, when a new active segment (501) resulting from ashift includes the left side of the left end (521) of the buffer B, asrepresented by an active segment (B4), another active segment (503) forthe substitution of speech data are again determined by the followingprocedure.

First, the active segment is shifted from the active segment (501) tothe right by one waveform period. Subsequently, whether or not the rightend (504) of the resulting new active segment (502) lies in the range ofone latest waveform period of the buffer B. If the answer of thisdecision is positive, then synthetic speech data for substitution areproduced by use of the active segment (502). If the answer of the abovedecision is negative, the active segment is further shifted to the rightby another waveform period in order to repeat the same decision. Such aprocedure is repeated until the right end of the shifted active segmententers one latest waveform period.

More specifically, to determine the start point of the active segment(503) newly selected, the end point of the previous frame issequentially shifted to the right by one waveform period at a time untilthe start point enters the active segment (503) as in the previousembodiment.

When the erasure of speech data continues even after the frame statedabove, the active segment (503) is sequentially shifted to the left, asrepresented by an active segment (511).

As stated above, the illustrative embodiment is adapted to allow asynthesized speech to vary even when a long erasure frame isencountered. This is accomplished by the structure preventing an activesegment from being consecutively involved in a particular range. Thisgives rise to maintaining the naturality in a synthesized speechreproduced, and preventing an undesired tonal sound to be output whichwould otherwise be caused by repetitive single waveforms.

Reference will be made to FIG. 6 for describing another alternativeembodiment of the speech erasure compensating circuitry in accordancewith the present invention. The illustrative embodiment is alsoidentical with the embodiment described with reference to FIGS. 3 and 4except for the method of determining an active segment when the activesegment shifted to the left by the shift period exceeds the range of thebuffer B. FIG. 6 shows the buffer B and the variation of the activesegment particular to the illustrative embodiment. Active segments (B1)through (B3) shown in FIG. 6 are identical with the active segments (B1)through (B3) shown in FIG. 4.

As shown in FIG. 6, when an active segment (601) newly determined by theleftward shift includes the left side of the left end (641) of thebuffer B as represented by an active segment (B4), the active segment(601) is shifted to the right by one waveform period, and the resultingsegment (602) is determined to be the active segment of the frame. Ifthe temporary start point lies in the active segment (602), it isdetermined to be the start point of the active segment (602) as in theprevious embodiment; otherwise, the temporary start point is shifted tothe right by one waveform period and then used as a start point. Therightward shift is repeated when the erasure continuously occurs in thesubsequent frames.

When an active segment (631) resulting from the repeated rightward shifteffected on a shift period basis includes the right side of the rightend (642) of the buffer B, a new active segment (632) is selected byshifting the active segment (631) to the left by one waveform period tothereby generate synthetic speech data. The start point (634) in theactive segment (632) is determined in the same fashion as in theprevious embodiment although the direction is opposite. When the erasurecontinuously occurs in the subsequent frames, the leftward shift of theactive segment is repeated by the shift period at a time. The proceduredescribed above is repeated until the erasure ends.

As stated above, the illustrative embodiment locates the active segmentsof nearby frames close to each other to thereby allow synthetic speechdata for substitution to be also close to each other with respect totime. This insures continuity between substituted waveforms in nearbyframes for thereby rendering transition between the frames natural.

Further, the illustrative embodiment is, like with the previousembodiment, so adapted to prevent an active segment from continuouslyexisting in a particular range, a substituted speech is renderedvariable. This prevents an undesired tonal sound to be reproduced thatwould otherwise be caused by repeating a single waveform.

Referring to FIG. 7, a further alternative embodiment of the speecherasure compensating circuitry will be described in accordance with thepresent invention. The illustrative embodiment is also identical withthe embodiment described with reference to FIGS. 3 and 4 except for themethod of determining an active segment when the active segment shiftedto the left or right by the shift period exceeds the range of the bufferB. FIG. 7 shows the buffer B and the variation of the active segmentparticular to the illustrative embodiment. Active segments (B1) through(B3) shown in FIG. 7 are identical with the active segments (B1) through(B3) shown in FIG. 4.

As shown in FIG. 7, when an active segment (701), selected by shiftingthe immediately preceding active segment (711), includes the left sideof the left end (741) of the buffer B, as represented by an activesegment (B4), the active segment (701) is shifted to the right until theleft end (703) of the segment (701) coincides with the left end (741) ofthe buffer B. The resulting new segment (702) is used as an activesegment for the generation of synthetic speech data. As for the startpoint in the segment (702), the temporary start point is determined tobe the start point if lying in the segment (702), or is otherwiseshifted to the left by one waveform period as in the procedure shown inFIG. 4.

When the erasure continues even in the successive frames, the rightwardshift of the active segment is repeated by the shift period at a time. Astart point in each active segment is determined by the same method asin the procedure of FIG. 6.

When an active segment (731) resulting from the rightward shift includesthe right side of the right end (742) of the buffer B, as represented byan active segment (B7), the segment (731) is shifted to the left untilthe right end (733) of the segment (731) coincides with the right end(742) of the buffer B. An segment (732) determined by such a leftwardshift is used as an active segment for the generation of syntheticspeech data.

Again, when the erasure continues even in the successive frames, theleftward shift of the active segment is repeated by the shift period ata time. A start point in each active segment may also be determined bythe same method as in the procedure of FIG. 6.

When erased frames continuously occur over a long period of time, theillustrative embodiment can use the entire range of speech data saved inthe buffer B for the generation of substitutive speech data without failand can therefore output substituted speech that sounds natural. Theillustrative embodiment is easily practicable with a memory having asmall capacity.

Further, the illustrative embodiment allows the waveform of substitutedspeech to contain the variation of the entire buffer B and, at the sametime, obviates an undesirable tone ascribable to a single continuouswaveform.

FIG. 8 demonstrates a conventional speech erasure compensating methodusing an internal memory 800 whose capacity is large enough to storespeech data over, e.g. up to three waveform periods. The speech datathus stored in the memory 800 are used to obviate a tone ascribable to asingle continuous waveform. This method, however, scales up the memory800 and access configuration thereof, increasing the scale of the entirecompensating circuitry.

Moreover, in accordance with the method of FIG. 8, when erased framescontinuously occur, an segment to be used for the generation ofsynthetic speech data are extended on a waveform period basis.Consequently, for consecutive erased frames, speech data for thegeneration of speech data are collected from a broad range, tending todegrade the natural variation of substituted speech.

By contrast, the illustrative embodiments of the present invention shownand described shift the position of speech data for gradual substitutionfor thereby shifting an segment to be used. The erasure of a speechsignal can therefore be compensated without lowering signal qualitydespite that speech data are not saved over three waveform periods.

While the illustrative embodiments have been shown and described asdetermining a shift period at all times, a shift period may not bedetermined in some circumstances, in which case the conventionalcompensation procedure will be executed. For example, if an erased frameis representative of an unvoiced segment whose correlation is small, asdetermined by the comparison of a difference between autocorrelationvalues and a preselected threshold or the comparison of a ratio betweenautocorrelation values and a preselected threshold, by way of example,then a shift period may not be determined.

The illustrative embodiments select, among periods shorter than awaveform period, a period having the largest autocorrelation value as ashift period. Alternatively, there may be selected, among a plurality ofamounts or periods of shift having autocorrelation values larger than apreselected value, a period closest to or farthest from a waveformperiod.

If desired, a single shift period determined in the illustrativeembodiments may be replaced with a plurality of shift periods. Forexample, a shift of an active segment using a first shift period and ashift of the same using a second shift period may be alternatelyeffected. Further, random numbers may be selectively used for eachshift.

While an active segment used in the illustrative embodiments iscoincident with a waveform period, the active segment may be providedwith a frame length or similar fixed length, in which case the shiftperiod must be shorter than the active segment. Even when the activesegment is fixed, a start point in an active segment after a shift isdetermined by use of the waveform period.

In the illustrative embodiments, overlap processing is suitably executedin the event of substitution. It should also be noted that theillustrative embodiments are applicable not only to a speech signalshown and described, but also to any other periodic signal, e.g. a musicsignal or a signal having a sinusoidal waveform.

In summary, it may have be seen that the present invention providescircuitry capable of substituting for erased part of a periodic signalwithout degrading signal quality.

The entire disclosure of Japanese patent application No. 2003-136338filed on May 14, 2003, including the specification, claims, accompanyingdrawings and abstract of the disclosure is incorporated herein byreference in its entirety.

While the present invention has been described with reference to theparticular illustrative embodiments, it is not to be restricted by theembodiments. It is to be appreciated that those skilled in the art canchange or modify the embodiments without departing from the scope andspirit of the present invention.

1. Compensating circuitry for substituting for erased periodic signaldata periodic signal data input before the erased periodic signal data,comprising: a past data saving circuit configured to save apredetermined number of latest periodic signal data input; a decisioncircuit configured to determine whether or not an erasure occurs withevery periodic signal data sequence, which is a unit of processing; asubstituting circuit configured to use, when an erasure occurs, aperiodic signal data sequence lying in a predetermined segment to beused among periodic signal data sequences saved in said past data savingcircuit, to generate synthetic data for substitution; and a positioncontroller configured to determine, when the erasure has occurred over aplurality of units of processing, a position of the segment to be usedsuch that the position varies for each of the units of processing. 2.The circuitry in accordance with claim 1, wherein said positioncontroller calculates periods of the periodic signal data sequencessaved in said past data saving circuit and selects, among the periodscalculated, a waveform period having highest periodicity as a width ofthe segment to be used.
 3. The circuitry in accordance with claim 1,said position control circuit calculates periods of the periodic signaldata sequences saved in said past data saving circuit and selects, amongthe periods calculated, a period shorter than a width of the segment tobe used as an index for varying the segment for every processing frame.4. The circuitry in accordance with claim 1, wherein said positioncontroller sequentially shifts the position of the segment to be usedfrom a newest periodic signal data sequence toward an oldest periodicsignal data sequence saved in said past data saving circuit anddetermines, when the segment cannot be further shifted toward the oldestperiod signal data sequence, the segment at a position adjacent to theoldest periodic signal data sequence.
 5. The circuitry in accordancewith claim 1, wherein said position controller sequentially shifts theposition of the segment to be used from a newest periodic signal datasequence toward an oldest periodic signal data sequence saved in saidpast data saving circuit, again sequentially shifts, when the segmentcannot be further shifted toward the oldest period signal data sequence,the segment from the newest periodic signal data sequence toward theoldest period signal data sequence, and repeats a variation effected bya shift so long as the erasure continues.
 6. The circuitry in accordancewith claim 1, wherein said position controller sequentially shifts theposition of the segment to be used from a newest periodic signal datasequence toward an oldest periodic signal data sequence saved in saidpast data saving circuit, sequentially shifts, when the segment cannotbe further shifted toward the oldest period signal data sequence, thesegment from the oldest periodic signal data sequence toward the newestperiod signal data sequence, sequentially shifts, when the segmentcannot be further shifted toward the newest periodic signal datasequence, the segment from the newest periodic signal data sequencetoward the oldest periodic signal data sequence, and repeats a variationeffected by a shift so long as the erasure continues.
 7. The circuitryin accordance with claim 1, wherein the periodic signal comprises aspeech signal.
 8. A compensating method for substituting for erasedperiodic signal data periodic signal data input before the erasedperiodic signal data, comprising: a past data saving step of saving apredetermined number of latest periodic signal data input; a decidingstep of determining whether or not erasure occurs with every periodicsignal data sequence, which is a unit of processing; a substituting stepof using, when an erasure occurs, among periodic signal data sequencessaved in said past data saving step, a periodic signal data sequencelying in a predetermined segment to be used to generate data forsubstitution; and a position controlling step of determining, when theerasure has occurred over a plurality of units of processing, a positionof the segment to be used such that the position varies for each of theunits of processing.
 9. The method in accordance with claim 8, whereinsaid position controlling step calculates periods of the periodic signaldata sequences saved in said past data saving step and selects, amongthe periods calculated, a waveform period having highest periodicity asa width of the segment to be used.
 10. The method in accordance withclaim 8, said position controlling step calculates periods of theperiodic signal data sequences saved in said past data saving step andselects, among the periods calculated, a period shorter than a width ofthe segment to be used as an index for varying the segment for everyprocessing frame.
 11. The method in accordance with claim 8, whereinsaid position controller sequentially shifts the position of the segmentto be used from a newest periodic signal data sequence toward an oldestperiodic signal data sequence saved in said past data saving step anddetermines, when the segment cannot be further shifted toward the oldestperiod signal data sequence, the segment at a position adjacent to theoldest periodic signal data sequence.
 12. The method in accordance withclaim 8, wherein said position controlling step sequentially shifts theposition of the segment to be used from a newest periodic signal datasequence toward an oldest periodic signal data sequence saved in saidpast data saving step, again sequentially shifts, when the segmentcannot be further shifted toward the oldest period signal data sequence,the segment from the newest periodic signal data sequence toward theoldest period signal data sequence, and repeats a variation effected bya shift so long as the erasure continues.
 13. The method in accordancewith claim 8, wherein said position controlling step sequentially shiftsthe position of the segment to be used from a newest periodic signaldata sequence-toward an oldest periodic signal data sequence saved insaid past data saving step, sequentially shifts, when the segment cannotbe further shifted toward the oldest period signal data sequence, thesegment from the oldest periodic signal data sequence toward the newestperiod signal data sequence, sequentially shifts, when the segmentcannot be further shifted toward the newest periodic signal datasequence, the segment from the newest periodic signal data sequencetoward the oldest periodic signal data sequence, and repeats a variationeffected by a shift so long as erasure continues.
 14. The method inaccordance with claim 8, wherein the periodic signal comprises a speechsignal.